WDM is a technique in fiber-optic transmission for using multiple light wavelengths (colors) to send data over the same medium, or to transmit two or more colors of light on one fiber, or to simultaneously transmit several signals in an optical waveguide at differing wavelengths. WDM systems involve a number of channels of different wavelengths being transmitted simultaneously in an optical fiber, thereby permitting the transmission capability of the optical fiber to be upgraded to the multiGbit/s range.
The term “fiber-optic” or “optical fiber” refers to an optical waveguide, typically comprising a core and a cladding, which is capable of carrying information in the form of light. The word “light” refers to electromagnetic radiation of any wavelength including laser and IR. The word light includes the portion of the electromagnetic spectrum that can be handled by a fiber-optic used for the visible spectrum extending from the near ultraviolet region of approximately 0.3 micron, through the visible region and into the mid-infrared region of about 30 microns.
A channel is a communication path. Multiple channels can be multiplexed over a single fiber in certain environments. The term “channel” is also used, in the case of fiber optic-based transmission systems, for an electrical or photonic communications path, between two or more points of termination. A wavelength is the length of one complete wave of an alternating or vibrating phenomenon, generally measured from crest to crest, from trough to trough of successive waves. For an electromagnetic waveform, a wavelength is the distance between two crests of the electromagnetic waveform.
Each channel typically is defined by a laser, or high brightness infrared (IR) light emitting diode (LED) source. A LED is a device used in a transmitter to convert information from electric to optical form. It typically has a large spectral width. A LED could be a semiconductor diode that emits light when forward biased to an optical signal.
A tunable optical device is a device for wavelength selection such as in an add/drop multiplexer (ADM), i.e., to add or drop a particular wavelength from a range of wavelengths, by applying a signal, e.g., an electrical signal, to the filter. An example of a tunable optical device is a tunable optical filter. The term ADM refers to a device that enables data to enter and leave an optical network bit stream without having to demultiplex the stream. Demultiplexing is a process applied to a multiplexed signal, i.e., a combination of several signals through a single communications channel, for recovering signals combined within it and for restoring the distinct individual channels of these signals.
In an optical filter, gratings are used to isolate a narrow band of wavelengths. In particular, grating reflectors are used to add or drop a light signal, i.e., filter a light signal, at a predetermined center wavelength to or from a fiber optic transmission system without disturbing other signals at other wavelengths. An optical tunable filter includes precision optical filters that can be tailored specifically for each wavelength of a light signal comprising a number of wavelengths.
A tunable optical filter produces a change in a bulk index of the filter material of the filter, i.e., the filter material, with a change of the signal applied across the filter, and hence of the wavelength of light transmitted through or diffracted from the filter is changed. For example, a hologram is recorded that diffracts light at a certain wavelength with no voltage applied and by changing the voltage across the filter, the refractive index of the filter material is changed. Hence, changing the voltage across the filter changes the wavelength that is diffracted.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. Electroholographic approaches have been studied and are being developed for making optical tunable filters. The prior art approaches use expensive photorefractive crystals and large voltages to change the bulk index of the filter material and hence the wavelength of light. The disadvantage of such an approach is that the crystals are expensive, suspect to optical and electrical damage, require large voltages and even then do not have a large bulk index change, which therefore limits the tuning range of the filter. In addition, these material support holographic gratings that are weaker (smaller index perturbation) than what can be achieved in polymer based materials.
Other prior art approaches use temperature and strain in material or mechanically stretch the filter material to alter the dimensions of the gratings to get tunable filters. Strain can change the optical quality of the filter materials, but limits the size, thickness, and mechanical properties of the filter material (i.e., to fibers). Using the temperature effect for controlling a tunable filter has many disadvantages. First, temperature is a very poor control signal because its response time is very slow. Second, a change in the environment temperature affects the tuning of the filter. Third, temperature is hard to accurately control for a small change in temperature. Because of the correction needed for variations in the environmental temperature, most of the temperature-based systems need a sensitive servo feedback path.
Therefore, there exists a need for an inexpensive tunable optical filter having a wide tuning range that can be easily and accurately controlled. The filter of this invention can be made inexpensively and has a wide spectral tuning range.